Introduction
Acid is one of the most distinctive damage types. It have big DoT damage, lots of offensive procs. But in current state of the game is considered weak damage type. According to @sir_spanksalot is the weakest damage type in the game. In my opinion it’s not. But frankly if we look the Acid builds, we’ll see that rarely they perform good. Only exceptions are Vileblade WH and Dervish. But even they are squishy. Also Dunefiend EoR is adequate but Shadow Strike side is weak to function. On the flip side Deathguard set is awful, same for Dreeg and Radaggan. Dark One is better suited for vitality damage. Radaggan and Deathguard in particular just lacks stats big time. RG even lacks RR.
Acid problems
So what are the big problems to Acid? First and foremost is DA. When I create every acid build, except VB Dervish I have laughable DA. Usually in the range of 2.6k-2.7k and that’s with BiS gear. Deathguatd full 4 pieces set for example gives OA and DA like single yellow affix-Readiness. But also your devotions and support gear don’t have enough of it.
Second problem is the absolute absence of CC resistances except Ascension, if you select Oathkeeper. But if you’re poor Cabalist, With Hunter or Reaper you’re screwed. Acid gear/devotions needs to offer some CC in order to stay competitive.
Third problem is the resistances types you get or the lack of diversity there. Almost every single item you want to use have acid RR. You want fancy Oathkeeper belt-acid RR, Pestilence of Dreeg-same, Meditation -acid resistance, boots -acid res both Stonetreader and Venomspine, also blue Serpentine. So why is problem? Blood of Dreeg usually render useless acid resistance but even if you don’t pick Occultist on acid builds you have 50-60% overcap pre augments and sometimes even pre components picking. And maybe the best armor component-Udgenbog leather gives acid resistance too. At same time you struggle with multiple other resistances types.
Some secondary problems can be low health(Witch Hunters), low Armor(non RF builds) and even low OA, since some sets/items don’t provide it. Also casters don’t have access to spirit, so can be problematic to wear off hands.
Solution
Not sure how to improve Acid outside of the underperforming sets. But some thoughts:
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Nidalla pants- add Stun or Slow resistance.
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Vilescorn gloves- add cast speed too.
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Deatguard ring- double the DA
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Eternal Rot- add disrupt protection.
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Venomspine- replace Acid with Slow/Stun.
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Add somewhere freeze resistance.
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Devotions, boost slightly stats like OA/DA on acid constellations.
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Add spirit to Aspect of Guardian or Possesion.
I know Acid RR on Murmur was increased so it’s fine. But that shows you that Acid damage needs slight boost to be more diverse damage type and function better. 
